As an alternative to forming poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) and other useful aliphatic polycarbonates using phosgene solution polymerization, basic alkali metals and their derivatives have been used as polymerization catalysts in melt and solid state polymerization processes. It has been found, however, that these aliphatic polycarbonates are somewhat thermally unstable because of residual basic alkali metal polymerization catalysts contained in the polycarbonate.
Various methods of adding acidic material to destroy the basic catalyst residues which remain in polycarbonates and cause instability have been reported in the literature. For example, U.S. Pat. No. 3,022,272 discloses that materials such as aromatic sulfonic acid, organic acid halides and chlorocarbonates, dialkyl sulfates, and acid salts of inorganic acids such as ammonium sulfate are useful "catalyst killers."
These acidic materials that have been used heretofore for this purpose have significant disadvantages. The aromatic sulfonic acids, acid halides and, to a lesser extent, the acid salts of inorganic acids, are corrosive and some, particularly the strong acids such as toluenesulfonic acid, have been found to produce undesirable color in the finished product. The dialkyl sulfates, while not so corrosive, are known to be both toxic and possibly carcinogenic so that their handling in practice is hazardous, and the presence of traces of them in the final product would present a problem for the consumer, especially if the plastic were used in food or cosmetic containers.
Another problem with many known deactivation methods is that the catalyst is not completely deactivated. Evidence of remaining active catalyst is shown in U.S. Pat. No. 3,022,272 which discloses that, after deactivation of the catalysts, the interesterification can be further continued to a limited extent in order to further increase the molecular weight of the polymer.
U.S. Pat. No. 2,210,617 discloses a process for preparing a polycarbonate in the presence of an alkali metal catalyst. After polymerization, the excess alkali metal is removed by washing with a strong acid such as hydrochloric acid. The polymerization is then completed by further heating under vacuum.
U.S. Defensive Publication T873,016 discloses removal of basic alkali metal catalyst residues from poly(2,2,4,4-tetramethyl-1,3-cyclobutylene carbonate) in the solid state or in solution. The process therein is conducted by contacting the polymer with an acidic organic compound with an ionization constant of about 2.times.10.sup.-1 to 2.5.times.10.sup.-6, followed by extraction either with the acid, or by dissolving the polymer in a water-immiscible solvent and extracting the solution with water.
In light of the above, it would be desirable to have a process for increasing the thermal stability of polycarbonates containing active alkali metal catalysts by completely deactivating the catalyst residue contained in the polymer. It would be especially desirable to provide a catalyst deactivation method which does not require the use of strongly acidic, corrosive, or volatile materials, nor washing or extraction of the catalyst from the polymer.